In oil-filled electrical equipment such as an oil-filled transformer, coil copper as a conducting medium has coil insulating paper wound therearound so that a structure for preventing occurrence of short-circuit between adjoined turns is provided.
Meanwhile, mineral oil used in the oil-filled transformer contains a sulfur constituent, and the sulfur constituent reacts with coil copper arranged in oil, so that conductive copper sulfide is generated. In the case where this copper sulfide is generated on a surface of insulating paper provided on the coil, a conduction path is formed from a point at which copper sulfide is deposited because copper sulfide is a conductive substance. Consequently, there has been known disadvantages such as occurrence of electric breakdown due to short-circuit of adjoining coil turns (for example, NPD 1 (CIGRE WG A2-32, “Copper sulphide in transformer insulation,” Final Report Brochure 378, 2009)).
Moreover, it has been known that a causative substance causing generation of copper sulfide is dibenzyldisulfide (DBDS) which is a kind of a sulfur compound in oil (for example, NPD 2 (F. Scatiggio, V. Tumiatti, R. Maina, M. Tumiatti, M. Pompilli and R. Bartnikas, “Corrosive Sulfur in Insulating Oils: Its Detection and Correlated Power Apparatus Failures”, IEEE Trans. Power Del., Vol. 23, pp. 508-509, 2008)).
It has been known that copper sulfide is generated on coil insulating paper by a process in which DBDS reacts with coil copper to generate a complex, a process in which the complex is diffused in oil to adhere to coil insulating paper, and a process in which the adhered complex is dissolved to become copper sulfide (for example, NPD3 (S. Toyama, J. Tanimura, N. Yamada, E. Nagao and T. Amimoto, “Highly Sensitive Detection Method of Dibenzyl Disulfide and the Elucidation of the Mechanism of Copper Sulfide Generation in Insulating Oil”, IEEE Transactions on Dielectrics and Electrical Insulation, Vol. 16, No. 2, pp. 509-515, 2009.)).
There has been a known method for suppressing copper sulfide generation by suppressing the reaction between DBDS and coil copper in accordance with the generation mechanism described above, and a method of adding an inhibitor to electric insulating oil is widely used. As an inhibitor of copper sulfide generation, 1,2,3-benzotriazole (BTA) or Irgamet39 is used (for example, NPD4 (T. Amimoto, E. Nagao, J. Tanimura, S. Toyama and N. Yamada, “Duration and Mechanism for Suppressive Effect of Triazole-based Passivators on Copper-sulfide Deposition on Insulating Paper”, IEEE Transactions on Dielectrics and Electrical Insulation, Vol. 16, No. 1, pp. 257-264, 2009.)).
When an inhibitor of copper sulfide generation is added to oil, the inhibitor reacts with coil copper to form a coat on a surface of the coil copper (for example, PTD 1 (Japanese Patent Laying-Open No. 6-76635)). Since this formed coat blocks or suppresses reaction between DBDS and coil copper, copper sulfide generation can be suppressed (for example, NPD 4).
However, in the case where the inhibitor for suppressing copper sulfide generation is added to insulating oil, since the coat formed by the reaction between the inhibitor and coil copper is gradually peeled off due to oxidative degradation or thermal degradation, there is a possibility that the inhibitory effect of the coat is faded (for example, NPD 4). Meanwhile, although DBDS in insulating oil is also consumed by the thermal degradation, it is rarely consumed at the actual equipment operation temperature (for example, NPD 5 (Maria Augusta G. Martins and Ana R. Gomes, “Experimental Study of the Role Played by Dibenzyl Disulfide on Insulating Oil Corrosivity-Effect of Passivator Irgamet 39”, IEEE Electrical Insulation Magazine, Vol. 26, No. 4 pp. 27-31, 2010.)). When the inhibitory effect of the coat is lost, the reaction between DBDS and coil copper recurs as long as DBDS is detected in insulating oil, thus copper sulfide is generated on insulating paper disadvantageously.
With respect to newly established equipment, when insulating oil not containing DBDS is used, a possibility of generation of copper sulfide is extremely low without adding the inhibitor. On the other hand, in the case where the inhibitor is applied with respect to existing equipment, there is a possibility that copper sulfide is generated immediately after the inhibitor is depleted. Therefore, there has been a problem of clarifying a criterion for determining a timing at which the inhibitory effect of the inhibitor is lost.